Keeping SQL tuned up helps performance of TFS.  Checking the fragementation levels of indexs within the SQL database is straightforward and SQL includes useful defragment commands.   Here is a link to a useful best practices document on defragmentation.  It was written for SQL 2000, but is applicable to 2005:

http://www.microsoft.com/technet/prodtechnol/sql/2000/maintain/ss2kidbp.mspx